This invention concerns an improved process for the production of hexabromocyclododecane. The process produces a low-melting, highly pure hexabromocyclododecane product.
Hexabromocyclododecane (1,2,5,6,9,10-hexabromocyclododecane) is a well established flame retardant for use in various thermoplastics. This compound is usually produced as a mix of its three stereoisomers, the alpha, beta and gamma isomers. It is preferred that the gamma isomer comprise about 70 wt % of the mix. Hexabromocyclododecane is commercially available as a product which contains principally hexabromocyclododecane and lesser amount of impurities. A principal impurity is the underbrominated species, tetrabromocyclododecene. The tetrabromocyclododecene impurities can comprise up to 10 wt % of the hexabromocyclododecane product. Lesser impurities include the side-reaction products formed by the reaction of the brominating agent with cyclododecatriene, brominated cyclododecane and reactive solvents, e.g., alcohols.
The product is produced by the bromination of cyclododecatriene in the presence of a solvent, generally an alcohol, e.g., isobutanol. The alcohol can be used alone or in combination with a wide variety of co-solvents, e.g., halogenated hydrocarbons, dioxane, etc. The main drawbacks with using alcohol-based solvents are that (i) a significant amount of the reaction intermediate, tetrabromocyclododecene, precipitates out of the reaction solution before the intermediate has an opportunity to be hexabrominated, and (ii) alcohols readily react with the brominating reagent to produce undesirable side-reaction products and to consume brominating reagent. If the so-consumed brominating agent is not replaced, increased formation of underbrominated species, e.g., tetrabromocyclododecene, is likely to occur. The industry has responded to these drawbacks by suggesting that the reaction mass could be heated, in a finishing step, to redissolve the tetrabromocyclododecenes and then further brominating the tetrabromocyclododecenes to form the hexabromocyclododecane. However, heating of the reaction mass exacerbates the formation of undesirable side-reaction products derived from the solvent. To increase the purity of the finished hexabromocyclododecane product, the crude process product is repeatedly washed with solvents, such as toluene, which will remove tetrabromocyclododecene and other impurities. Unfortunately, such washing will also remove significant amounts of the alpha and beta isomers which reduces the total yield of hexabromocyclododecane product. In addition, since the alpha and beta isomer content is reduced without a concomitant reduction in the gamma content, the melting point of the hexabromocyclodecane product will be high, say between 185 to 200.degree. C. Such high-melting products are commercially viable, however, the more commercially significant hexabromocyclododecane products are those having a melting point between 175 to 195.degree. C., which products are referred to by the industry as low-melt products. Low-melt products generally contain 10 to 12 wt % alpha isomer, 4 to 9 wt % beta isomer and 71 to 78 wt % gamma isomer. But the problem with most low-melt products is that they have a high impurity content since they have not undergone the washing steps, indeed they contain up to about 10 wt % tetrabromocyclododecene. With a high impurity content, comes a less than desirable thermal stability.
It is an object of this invention to provide a process for the production of low-melt hexabromocyclododecane products having good thermal stability. This and other objects will be more fully understood from the following description of various processes and products which are claimed herein.